It is conventional practice in color television and allied display applications, to utilize cathode ray tubes of the type employing a patterned multi-phosphor cathodo-luminescent screen; such being interiorly disposed on the viewing panel portion of the tube envelope, wherein an apertured or multi-opening mask member is spatially positioned relative thereto. A plurality of electron beams, emanating from an electron gun assembly encompassed within the neck portion of the tube envelope, are directed to converge at and traverse the apertured mask to thence impinge and luminescently excite the array of electron responsive phosphors comprising the patterned screen therebeyond. Focusing of the respective electron beams is conventionally achieved by discrete electron lensing means; as for example, tri-potential focus lensing, such being accomplished by a distributed focusing lens system embodying four sequential electrodes (G3), (G4), (G5), (G6) whereupon three different potentials (low, intermediate and high), are suitably applied.
The development of cathode ray tube technology has resulted in a marked trend toward miniaturization and compaction of electron gun structures, which in turn, are encompassed within envelope neck portions of smaller diameters and shorter lengths. Consequently, the dimensionings and constructions of the electrode elements of the multi-gun assemblies have been adapted to achieve the desired compaction. This is especially evident in the conventional inline plural gun assembly, wherein three separate electron beams are projected in a substantially common horizontal plane. Such is expeditiously accomplished by employing a unitized construction in which several of the respective electrode elements, ahead of the individual cathodes, are unitized electrode members of definitive construction, each having at least one plane with three spatially-related inline apertures therein.
In addition to gun assembly miniaturization, there has been wide acceptance of self-converging deflection yokes, fostered mainly by their improved performance, manufacturing efficiency and cost advantages. To more fully realize the advancements afforded by the self-converging system, the "shaped beam" concept has been introduced into electron gun construction to provide a significant improvement in deflected spot shape, thereby achieving a better focus balance between the center and corners of the screen.
The term "shaped beam", as referred to herein, is a beam or bundle of electron rays having a defined cross-sectional body configuration of a shape other than circular. In this instance, the beam, in its passage through the gun structure, is substantially of ovate shaping. The beneficially shaped beams are usually effected in the primary portion of the gun structure, for example, by the control grid (G1) and/or screen grid (G2) electrodes wherein aperture-related beam shaping configurations impart discrete cross-sectional shaping to the bundle of moving electrons passing therethrough. Exemplary art relating in general to beam shaping electrode structures is evidenced in filed U.S. patent applications Ser. No. 094,405, now U.S. Pat. No. 4,307,498, Ser. No. 094,409, now U.S. Pat. No. 4,272,700 Ser. No. 094,515 now U.S. Pat. No. 4,251,747 and Ser. No. 175,165, all of which are assigned to the assignee of the present invention.
In a unitized tri-potential in-line plural beam electron gun assembly one or all three of the undeflected focused beam spot landings at the center of the screen may be of ovate shaping. This is thought to be due to asymmetrics in the focusing field of the lensing in the gun, or to asymmetrics in the beam itself as it passes through the principal lensing region. For instance, in a TPF tube utilizing "shaped beams" to improve deflected performance, the respective beams evidence horizontally elongated cross-sectional shapings as they pass through the lensing effected within the gun structure; and upon arriving undeflected at the center of the screen, tend to exhibit ovate beam spot landing areas of substantially vertical orientation rather than the desired circular spot landings. Such focused ovate shapings appear to be the result of structural influences inherent in the unitized TPF construction which introduce asymmetrics and spherical aberration into the lengthy shaped-beam lensing fields. These accentuate overfocusing in the horizontal plane. The resultant vertically oriented beam spot landing is much more pronounced for the beam projected by the center gun, and may additionally evidence horizontal tailings extending from either side thereof. Any degree of presence of such center-of-screen abnormalities, from any or all of the guns, tends to detract from the desired picture resolution in the screen display of a TPF tube.